"In the resting state the cell interior is negative relative to the cell exterior for about −70 mV."

−70 mV

Communication by neurons depends on two things. One of them
is an electrical voltage known as the resting membrane potential
(RMP) across the membrane. Since the inside of a cell is negative
relative to the outside, a voltage across the membrane results.

The voltage across the plasma membrane of cells in the body
is usually between −20 to −200 millivolts (mV). The negative sign
means that the inside is negative relative to the outside. The
greater the difference in charge across the membrane, the larger
the voltage. The resting membrane potential (RMP) for nerve cells
is from −40 mV to −90 mV. The most common value is −70 mV,
and the membrane is said to be polarized.

Ions carrying most of the current, flow across the membrane
through ion channels. Two factors contribute to the resting membrane
potential: the distribution of ions across the plasma membrane
and the relative permeability of the membrane to Na (Sodium) and
K (Potassium). This voltage is established when the Na that diffuses
in, is pumped out and the K that diffuses out is pumped back in.

When an action potential occurs two voltage-gated ion channels
open and close, one for Na and one for K. When the voltage-gated
channel of Na opens depolarization occurs. And the opening of
the K voltage-gated channels and closing of the Na voltage-gated
channels causes repolarization. Both of these phases occur in
only a few milliseconds. During an action potential the internal
voltage of the cell goes from its resting potential (−70 mV)
to a positive value. However, it goes back to its resting potential
right away.